1. Field of the Invention
The present invention relates to a distribution result verification method for use in an information distribution system. More particularly, the present invention relates to a distribution result verification method suitable for use with a system which distributes a large volume of information to a plurality of receiving stations, as in the case of a system which distributes motion pictures by way of a communications satellite.
2. Background Art
In a communications system requiring a high degree of reliability, in the event of a communications error, a sender station must re-send the data which a receiving station has failed to receive. To this end, in such a communications system, a receiver station sends, in reply to the sender station, a response signal indicating whether or not data have been successfully received. The sender station can detect occurrence of a communications error by means of presence/absence of the response signal or from contents of the response signal.
When data are simultaneously distributed to a plurality of receiver stations as in the case of a motion picture distribution service, the sender station must check whether or not all the receiver stations have received the data. In the case of data of great size such as data pertaining to motion pictures, the data are transmitted while being divided into unit data sets of appropriate sizes. Therefore, the sender station must check successful receipt of data on a per-unit-data-set basis. FIG. 4 shows a processing sequence employed in sending data on a per-unit-data-set basis. A sender station 1 simultaneously sends a unit data set to individual receiver stations 2. Subsequently, the sender station 1 sends, to the respective receiver stations 2 on an individual basis, an inquiry signal 7 for inquiring the result of data distribution. In response to the inquiry signal 7, each of the receiver stations 2 sends a response signal 8 indicating the result of receipt of data. By means of the sender station 1 making inquiries of the receiver stations individually, congestion of the response signals 8 at the sender station 1 can be decreased. In the illustrated example, receiver stations 21 and 22 have successfully received a unit data set 4, whereas a receiver station 2N has failed to receive the unit data set 4. In this case, the receiver stations 21 and 22 return, as the response signals 8, ACK signals indicating successful receipt of the unit data set 4. The receiver station 2N returns, as the response signal 8, a NACK signal indicating unsuccessful receipt of the unit data set 4. Similar processing is iterated on a per-unit-data-set basis. According to this method, the greater the number of receiver stations or the greater the size of data to be distributed, the longer a time required for checking the result of data distribution. This imposes great burden on processing performance of the sender station, thus hindering timely distribution of data. A method of solving this problem is described in Japanese Patent Application Laid-Open No.296122/1992. According to this method, a relay station sends, to the transmission station and as a single result, distribution results pertaining to several receiver stations. In a video communications system described in Japanese Patent Application Laid-Open No. 224863/1994, the sender station has sent, to receiver stations beforehand, a schedule of programs to be distributed. Receiver stations check received data against the program schedule, thus detecting occurrence of receipt failures. The only requirement is that the sender station transmits data again only when a re-transmission request is issued from a receiver station that has detected a receipt failure. Hence, the sender station does not perform any processing for checking distribution results. The preceding method shortens a time required for checking distribution results as compared with the related-art method. However, processing time of and a burden imposed on the sender station increase in proportion to the number of receiver stations and the size of data to be distributed. The latter method is susceptible to restrictions; that is, there is a necessity of distributing a program schedule to a receiver station beforehand. For this reason, the method is not suitable for use in an information distribution service for which a schedule has not been determined.